The present invention relates to a laser processing system used for performing surface treatment or welding of an underwater structure and a method using the system, and particularly to a laser processing system used for repairing a structure in a pressure vessel of a nuclear reactor in a state in which the structure is disposed in water, and a method using the system.
The interior of a nuclear reactor which has operated for a long period of time is high in the dose equivalent of .gamma.-rays, and accordingly, upon thermal repairing of a structure in a nuclear reactor, the apparatus and the operators must be prevented from being exposed to .gamma.-rays. For this reason, the repairing operation should be performed in a state in which the structure is disposed in water capable of shielding .gamma.-rays. Namely, it is desirable to perform the surface reforming and welding for a structure in a pressure vessel of a nuclear reactor in water. Even for bridges and ships, an effective technique for repairing a structure in water has come to be in high demand.
With respect to thermal processing, such as welding or surface treatment applied to an underwater structure, a wet-type thermal processing without removal of the water near a target portion is now in reliability because the target portion tends to generate defects, such as blow-holes, and thereby the quality is deteriorated. In particular, the repairing of a structure in a nuclear reactor by welding or surface treatment must be performed at a high reliability, and thereby a processing method capable of reducing defects is required. For these reasons, there has been proposed a local dry type processing method in which thermal processing is performed in the state in which water in a target portion is partially discharged.
The local dry type processing method, however, is insufficient in the case of using a laser as a thermal source. This is because a laser beam is scattered by hazing and dew formation of the laser emitting optics which are caused by a slight amount of water content remaining near the target portion, causing a reduction in the workability of the laser processing. As a result, for laser processing, it is important to reduce the water content in a chamber of a laser processing system as much as possible. In this regard, various techniques have been examined.
For example, techniques for locally excluding the water content in the neighborhood of a laser torch of a laser processing system used in water and on a target portion to be processed to effect drying of such portions, have been disclosed in Japanese Patent Laid-open Nos. Sho 63-215392, Sho 63-242483, Hei 3-146286, Hei 4-249799 and Hei 5-31591. These repairing techniques are effective to obtain a processed region having less defects as compared with the wet-type processing method.
The above-described techniques, however, do not take into account problems generated during the actual applications. Specifically, in the actual repairing of a structure in a nuclear reactor, the repairing operation is started after an underwater processing system is moved in the water to a target portion. However, in each of these techniques, part of a water shield (chamber) for preventing water from permeating into the interior of the system is opened to a water environment (that is, the shielding mechanism is not a perfectly closed system). Consequently, to prevent permeation of water when the system is moved in water, a water curtain must be formed usually by allowing gas or water to flow. In this case, it may be considered that permeation of water can be prevented by increasing the flow rate of water. However, to increase the flow rate of water, a generator for generating a high pressure gas or high pressure water flow is required, or the water shield (chamber) must have a high strength for withstanding the high pressure gas or high pressure water flow. For the system capable of coping with the above problem, there arises another problem. In the case of processing in a downward posture, for example, for repairing the bottom of a reactor, the permeation of water is not so large; however, in the case of processing the side surface of the interior of a reactor, the processing system must be located with a horizontal posture. In this case, permeation of water becomes larger. The reason for this is that, if the system is not sufficiently close to a wall facing the opened portion of the system, the high pressure gas tends to flow upward because of its light specific gravity, and thereby the formation of the water curtain on the lower side of the system becomes insufficient. Namely, for a horizontal processing using a conventional system, it must be assumed that the facing wall is sufficiently close to the system. For this reason, up on operation of the conventional processing system for a target portion of an underwater structure, there is a fear that permeation of water will occur in the system or that generation of hazing or dew formation will occur in the laser emitting optics.
A slight amount of water content, which has presented no problem in thermal processing using an arc, causes hazing or dew formation of the laser emitting optics of a laser processing system during laser processing. In particular, for processing, such as cutting, welding or surface-treatment using emission of a laser beam, the high energy of the laser beam heats the material to a high temperature, and thereby the laser torch or the surrounding portion thereof is also heated during processing. As a result, the difference in temperature between the exterior and the laser torch becomes large, causing dew formation during processing, thus causing an inconvenience due to the dew formation. Specifically, a laser beam is abnormally scattered due to dew formation on a laser lens, causing reduction in the condensing performance or failure of the system due to heating of the portion of the lens where the dew formation is generated.